This invention relates to a solid state image sensor and, in particular, to a solid state image sensor decreased in shading amount.
Shading has been an issue of conventional solid-state image sensors, and studies have been made for decreasing the shading amount. Shading pertains to the phenomenon that incident light applied with a uniform intensity to the entirety of a solid-state image sensor is actually received unevenly, with a certain attenuation amount especially at end portions of the solid-state image sensor, and the attenuation amount is called shading amount.
FIG. 6 is an explanatory diagram of such shading amounts.
As shown in FIG. 6, even when image signal light is uniformly applied in the period of 1H of image signals, for example, as shown by the solid line, the signal actually received by a solid-state image sensor may attenuate at end portions thereof as shown by the broken line. The amount of the attenuation is the shading amount. Possible causes of shading are, for example, unevenness of lenses and differences in optical pick-up rate between the center and ends of the solid-state image sensor.
FIG. 7 shows a conventional solid-state image sensor in a cross-sectional view.
The solid-state image sensor 1 includes microlenses 17a through 17c, smoothing layer 16, color filters 15a through 15c, insulation layer 14, shade films 13a through 13c, electrodes 12a through 12c, photodiodes 18a through 18c, and semiconductor substrate 11. Adjacent color filters 15a and 15b are appropriately selected to pass different colors. The microlens 17a, color filter 15a, shade film 13a, electrode 12a and photodiode 18a, for example, form a single pixel. Incident light enters through the microlens 17a, and a predetermined color contained in the incident light is filtered through the microlens 17a, then converted into an electric signal by the photodiode 18a, and transmitted by the electrode 12a, etc.
In the conventional solid-state image sensor, the microlenses 17a to 17c are disposed so that their pitch or center axes disagree with the pitch or center axes of the pixels.
FIG. 8 is a cross-sectional view for explaining a positional relation in the conventional solid-state image sensor designed to decrease the shading amount.
In FIG. 8, color filters 82-0, 82-2, 82-4 pass a first color whereas color filters 82-1 and 82-3 pass a second color different from the first color.
In a pixel in a central portion of the chip of the solid-state image sensor, the microlens 81-0 is disposed to locate its center in substantial agreement with the aperture center, namely, centers of the color filter 82-0 and the photodiode 83-0. On the other hand, in pixels in peripheral portions of the chip, the microlenses 81-1 through 81-4 are disposed to deviate their centers from the aperture centers toward the center or peripheries of the chip by shift amounts (offset amounts) d1 through d4 which progressively increase toward the chip ends. Here, pitches of the color filters 82-0 through 82-3 and photodiodes 83-0 through 83-4 forming pixels are constant in all pixels in the chip.
In FIG. 8, distances from the photodiodes 83-1 through 83-4 as aperture centers to centers of the microlenses 81-1 through 81-4 are substantially equal in adjacent pixels having different color filters, although slightly larger in peripheral portions of the chip.
In this manner, sufficient light can enter into photodiodes even when the angle of incidence of light is slanted.
In conventional technologies, the pitch of microlenses is uniform in any kinds of color pixels. Therefore, offset amounts of microlenses from aperture centers are substantially equal in pixels substantially equally distant from the chip center, regardless of the color pixels being of different types or not, even though there may be a slight difference.
However, since refractive indices of microlens are different depending on wavelengths, positions of focal points of microlenses relative to apertures of color pixels in peripheral portions of the chip are different among different kinds of color filters of the pixels. Therefore, conventional technologies cannot minimize shading amounts simultaneously for all color outputs.